The issue one is pretty good, it's a good size, has plenty of features and it's free. The only drawback is when using the shoulder straps, if the bag is too full and too heavy they cause the bag to tear near the top of the straps.
i got this one from cotswolds, and is a top piece of kit, for £36 after MoD discount is a good price. Looks very much a north face rip off but for half the price with just as good a quality i won't complain.
I find that the issue one is awesome and if you go diffy costs £6.32 ( Basic price - 50% + VAT). Can't fall off with that, and if they sold them in outdoor shops I bet you would pay £40+ or probably more if it was North Face or similar manufacturer.
Why not try one of the Gorilla boxes from a yank PX (if you have access to one!)? They have pretty much the same capacity of an issued holdall and can double up as a nice bedside table to make tour feel a bit more civilised. Plus the added bonus of being (almost) RAF mover-proof!
I bought a holdall from Blacks about 5-6 years ago made by a firm called ALS. Its still going strong now and has clocked up some astronomical air miles, plus its got wheels at one end for when I feel like a lazy barsteward!